1. Field of the Invention
This invention relates to a prism having a liquid container to be used for measurement, and more particularly to be used as an apparatus in which an angle of inclination is measured based on the direction of gravity.
2. Description of the Prior Art
Heretofore, there is known a prism and associated liquid container to be used for an inclination measuring apparatus in which a liquid such as silicon oil is sealed in a liquid container in order to always provide a free liquid surface perpendicular to the direction of gravity, irrespective of the inclination of the apparatus. There are two types of inclination measuring apparatus; one is a reflection type in which light is caused to enter in the liquid prism and then the deflection angle of light totally reflected on and radiated from the free surface of the liquid is measured, and the other is a transmission type of inclination measuring apparatus in which light enters the liquid prism and then the transmission deflection angle of light transmitted through the free surface of the liquid is measured.
However, when these two types of apparatus are used for measurement, a problem results when the outside temperature is suddenly changed, because this causes the reflection light or transmission light to be disordered for a long time, and during this time the measurement either cannot be made at all or cannot be made accurately. However, the cause for this problem is not clear, but it was was considered to be attributable to a defect in the liquid prism.
After a careful study, the present invention has found that with conventional liquid prisms, when a sudden change in temperature results, a temperature difference is produced between a portion of the liquid contacting the liquid container and a portion of the liquid in the center thereof. This causes a convection of the liquid, and as a result, the smoothness of the free liquid surface is deteriorated for a long time. That is, when the outside temperature is drops abruptly, the temperature of the liquid around the area contacting the container becomes lower than that of the liquid around the central area of the container. Thus, the gravity of the former becomes larger than that of the latter. Accordingly convection is produced wherein the liquid extends upwardly at its central portion and downwardly at its wall surface portion. The result is that the free liquid surface is not stable which causes the disorder of the light reflected on or transmitted through the surface. Similarly, when the temperature is suddenly raised, the reverse happens thus producing a similar phenomenon to that.